Depending on the experimental conditions, the solid-state reactions of CuI with pyrazine (pyz) yield three distinct coordination polymers (CPs): a double chain polymer [Cu 2 I 2 (pyz)] n (yellow powder) and a single strand [CuI(pyz)] n (red powder) and its new isomeric compound [(CuI) 2 (pyz) 2 ] n (orange powder), which present dimers of CuI bridged by the pyrazine ligands. The conversions among the three CPs were studied: by heating to 110 °C, [CuI(pyz)] n or [(CuI) 2 (pyz) 2 ] n convert into [Cu 2 I 2 (pyz)] n , which reverts to the starting compounds upon kneading or grinding in the presence of pyrazine. The orange isomer [(CuI) 2 (pyz) 2 ] n is obtained only when the solid-state reaction is performed with neat grinding or by direct melting of the pyrazine; it is unstable in the presence of solvent or vapor, and it readily transforms into the red isomer. The structure of [(CuI) 2 (pyz) 2 ] n was determined by X-ray powder diffraction. [Cu 2 I 2 (pyz)] n reacts also with 4-cyanopyrazine to yield the mixed ligand compound [(CuI)(4CNpy) 2 py] n , which, when heated, decomposes into [Cu 2 I 2 (pyz)] n and [(CuI) 4 (4CN-py) 5 ] n.
The impact of the processing method in controlling the polymorphism and field-effect charge mobility of 2,3-thienoimide-based oligothiophenes semiconductors was investigated.
Understanding of polymorphism of organic semiconducting materials is the key to structural control of their electrical and mechanical properties. Motivated by the ambipolar n-type charge transport and electroluminescence of thienopyrrolyldione end-capped oligothiophenes, here we studied the propensity of one representative to crystallize as different polymorphs which display distinctly different mechanically properties. The crystal structures of the two polymorphs (denoted “α” and “β”) of the material, 2,2′-(2,2′-thiophene-5,5′-diyl)bis(5-butyl-5H-thieno[2,3-c]pyrrole-4,6)-dione (C4-NT3N), were determined. In the α phase, the molecules interact strongly by π-stacking, forming columns which are bonded via C–HO and chalcogen bonds, and this packing is consistent with the elastic behavior observed with the crystals. Instead, the β phase has the molecules aligned along their core forming layers. While the molecules interact strongly within the layers, they are practically unbound between the layers. The presence of slip planes in this form explains the plastic deformation induced by applying a force perpendicular to the (001). The thermal behavior and the enantiotropic relationship of the polymorphs are reported.
Anionic complexes having vapochromic behavior are investigated: [K(H2O)][M(ppy)(CN)2], [K(H2O)][M(bzq)(CN)2], and [Li(H2O) n ][Pt(bzq)(CN)2], where ppy = 2-phenylpyridinate, bzq = 7,8-benzoquinolate, and M = Pt(II) or Pd(II). These hydrated potassium/lithium salts exhibit a change in color upon being heated to 380 K, and they transform back into the original color upon absorption of water molecules from the environment. The challenging characterization of their structure in the vapochromic transition has been carried out by combining several experimental techniques, despite the availability of partially ordered and/or impure crystalline material. Room-temperature single-crystal and powder X-ray diffraction investigation revealed that [K(H2O)][Pt(ppy)(CN)2] crystallizes in the Pbca space group and is isostructural to [K(H2O)][Pd(ppy)(CN)2]. Variable-temperature powder X-ray diffraction allowed the color transition to be related to changes in the diffraction pattern and the decrease in sample crystallinity. Water loss, monitored by thermogravimetric analysis, occurs in two stages, well separated for potassium Pt compounds and strongly overlapped for potassium Pd compounds. The local structure of potassium compounds was monitored by in situ pair distribution function (PDF) measurements, which highlighted changes in the intermolecular distances due to a rearrangement of the crystal packing upon vapochromic transition. A reaction coordinate describing the structural changes was extracted for each compound by multivariate analysis applied to PDF data. It contributed to the study of the kinetics of the structural changes related to the vapochromic transition, revealing its dependence on the transition metal ion. Instead, the ligand influences the critical temperature, higher for ppy than for bzq, and the inclination of the molecular planes with respect to the unit cell planes, higher for bzq than for ppy. The first stage of water loss triggers a unit cell contraction, determined by the increase in the b axis length and the decrease in the a (for ppy) or c (for bzq) axis lengths. Consequent interplane distance variations and in-plane roto-translations weaken the π-stacking of the room-temperature structure and modify the distances and angles of Pt(II)/Pd(II) chains. The curve describing the intermolecular Pt(II)/Pd(II) distances as a function of temperature, validated by X-ray absorption spectroscopy, was found to reproduce the coordinate reaction determined by the model-free analysis.
Drug development may include extensive screening for crystalline forms of active pharmaceutical ingredients. Crystal engineering aims to apply supramolecular knowledge to simplify such a task. The failure of such a strategy may result in overlooking potentially interesting compounds. Here, the advantages of such a knowledge-based approach is compared to a systematic crystallization screening for pharmaceutical cocrystals. This work indicates that a screening simply based on known synthons and their relative frequency as reported in the database is as effective as a random screening exercise, potentially missing 25% of the successful cocrystallization. Readily available computational methods perform better, enabling the identification of all of the observed cocrystals with a reduction of 24% of the experimental attempts.
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